linux-kernel - [PATCH bpf-next] bpf: Lock bpf_trace_printk's tmp buf before it is written to

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Message-Id: <20210427112958.773132-1-revest@chromium.org>
Date:   Tue, 27 Apr 2021 13:29:58 +0200
From:   Florent Revest <revest@...omium.org>
To:     bpf@...r.kernel.org
Cc:     ast@...nel.org, daniel@...earbox.net, andrii@...nel.org,
        kpsingh@...nel.org, jackmanb@...gle.com, linux@...musvillemoes.dk,
        linux-kernel@...r.kernel.org, Florent Revest <revest@...omium.org>
Subject: [PATCH bpf-next] bpf: Lock bpf_trace_printk's tmp buf before it is written to

bpf_trace_printk uses a shared static buffer to hold strings before they
are printed. A recent refactoring moved the locking of that buffer after
it gets filled by mistake.

Fixes: d9c9e4db186a ("bpf: Factorize bpf_trace_printk and bpf_seq_printf")
Reported-by: Rasmus Villemoes <linux@...musvillemoes.dk>
Signed-off-by: Florent Revest <revest@...omium.org>
---
 kernel/trace/bpf_trace.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
index 2a8bcdc927c7..0e67d12a8f40 100644
--- a/kernel/trace/bpf_trace.c
+++ b/kernel/trace/bpf_trace.c
@@ -391,13 +391,13 @@ BPF_CALL_5(bpf_trace_printk, char *, fmt, u32, fmt_size, u64, arg1,
 	if (ret < 0)
 		return ret;
 
+	raw_spin_lock_irqsave(&trace_printk_lock, flags);
 	ret = snprintf(buf, sizeof(buf), fmt, BPF_CAST_FMT_ARG(0, args, mod),
 		BPF_CAST_FMT_ARG(1, args, mod), BPF_CAST_FMT_ARG(2, args, mod));
 	/* snprintf() will not append null for zero-length strings */
 	if (ret == 0)
 		buf[0] = '\0';
 
-	raw_spin_lock_irqsave(&trace_printk_lock, flags);
 	trace_bpf_trace_printk(buf);
 	raw_spin_unlock_irqrestore(&trace_printk_lock, flags);
 
-- 
2.31.1.498.g6c1eba8ee3d-goog